Where did the story come from?

The researchers who did this study were from Tulane University in the US.

It was funded by the US National Institutes of Health and published in the British Medical Journal. The study is free to read online.

The study was covered with enthusiasm by the UK media. Several reports said glucosamine cut the risk of a fatal heart attack or stroke by a fifth.

While the figure is correct (relative risk of a fatal heart attack or stroke is 22% lower), it represents an absolute risk difference of less than 1%, from 0.7% of people to 0.5% of people dying. And we do not know whether glucosamine is the reason for the lower risk.

Most of the news stories included comments from experts advising that people focus on a healthy lifestyle rather than taking dietary supplements.

What kind of research was this?

This cohort study used data from the ongoing UK Biobank study.

Cohort studies are good ways to spot patterns between different factors, in this case glucosamine use and cardiovascular disease.

But they cannot tell us if one factor (glucosamine) causes another (lower risk of cardiovascular disease). Other factors may be involved.

What did the research involve?

The UK Biobank study enrolled 500,000 adult volunteers across the UK and took a mass of data about them, including lifestyle and dietary questionnaires, fitness tests and DNA tests.

People in the study are followed up through mortality and medical records, and questionnaires.

The researchers used data on 466,039 people who did not have cardiovascular disease at the start of the study and had answered questions about their use of glucosamine.

after taking account of possible confounding factors, people who took glucosamine were 15% less likely to have had a heart attack or stroke (hazard ratio [HR] 0.85, 95% confidence interval [CI] 0.80 to 0.90)

after taking account of possible confounding factors, this represented a 22% reduced risk of dying from a heart attack or stroke (HR 0.78, 95% CI 0.7 to 0.87) for people who took glucosamine

How did the researchers interpret the results?

The researchers said: "Habitual use of glucosamine supplement to relieve osteoarthritis pain might also be related to lower risks of CVD [cardiovascular disease] events. Further clinical trials are needed to test this hypothesis."

Conclusion

Headlines suggesting that a supplement can "slash" your risk of heart disease or stroke are attractive, but the study does not back up the headlines.

This type of study cannot prove that one factor (glucosamine) is the cause of the results they found.

While the researchers tried to take account of other possible causes of the results, they could not account for everything.

The study did not collect information about how long people had taken glucosamine, what dose they were taking or how often they took it, and did not check whether people carried on taking it after the start of the study.

It's also worth bearing in mind that the actual change in risk – even if it was caused by glucosamine – is quite small.

A 15% drop in risk is smaller than the drop you'd see if you gave up smoking, for example. Smoking increases the risk of getting heart disease by 24%.

As one of the experts quoted in the press today says, if you want to reduce your risk of having a heart attack or stroke, it would be much better to concentrate on living a healthy lifestyle, rather than paying for glucosamine supplements.

Steps such as not smoking, taking regular exercise and eating a healthy, balanced diet are known to reduce heart attack and stroke risk.